1. Field of the Invention
The present invention relates to a mold used for manufacturing a rotor of an electric motor.
2. Description of the Related Art
In a rotor of an electric motor having a permanent-magnet field system (e.g., a synchronous motor), it is known that a plurality of permanent magnets are mounted in an embedded manner at predetermined positions in a cylindrical rotor core formed by stacking or laminating magnetic sheets such as silicon steel plates. The rotor of this type is typically manufactured by preliminarily forming a plurality of magnet-retaining apertures (typically, axially-extending through holes) in a predetermined circumferentially-distributed arrangement about a rotor shaft, each aperture having a profile substantially corresponding to a profile of each permanent magnet; inserting respectively the plurality of permanent magnets into the magnet-retaining apertures; and fixing each permanent magnet by an adhesive or an impregnant.
In the above-described conventional method for manufacturing a rotor, in the case where the adhesive is used for fixing the magnets, relatively complicated manual operations, such as the application of the adhesive to the permanent magnets, the removal of the surplus adhesive leaking from the magnet-retaining apertures, and the like, may be required. Also, in the case where the impregnant is used for fixing the magnets, relatively large-scale equipment tends to be required for an immersion process for immersing the rotor core in the impregnant, a heating process for solidifying the impregnant, etc., and thus the number of manufacturing steps tends to increase. On the other hand, it is also known that, as a rotor manufacturing method capable of resolving the above inconveniences, the rotor core is previously fabricated so that predetermined gaps are formed between respective magnet-retaining apertures and respective permanent magnets, and a resinous material is locally pored into the gaps and solidified therein, so as to fix the permanent magnets to the rotor core (e.g., see Japanese Unexamined Patent Publication (Kokai) No. 5-83892 (JP-A-5-083892)).
In the rotor manufacturing method disclosed in JP-A-5-083892, grooves (i.e., gaps) extending in an axial direction are formed adjacent to and in communication with the respective magnet-retaining apertures of a rotor core, and a molten resinous material is injected into the grooves and solidified therein by an injection molding technique, so as to fix the permanent magnets in the magnet-retaining apertures. In the injection molding process, a mold including a cavity for accommodating the rotor core and an injection molding machine onto which the mold can be installed are provided, and the rotor core receiving the permanent magnets in the respective magnet-retaining apertures is supported firmly and securely in the cavity of the mold. In this state, the resinous material is injected into the grooves of the rotor core under pressure by using the injection molding machine.
In the rotor manufacturing method adopting the injection molding technique set forth in the above-described JP-A-5-083892, by only providing the mold including the cavity for accommodating the rotor core and the injection molding machine onto which the mold can be installed, it is possible to easily and quickly perform a work for fixing the permanent magnets to the respective magnet-retaining apertures of the rotor core and, therefore, advantages are obtained in which complicated manual operations when using an adhesive and large-scale equipment when using an impregnant are eliminated. However, in order to accurately inject the resinous material into the narrow grooves formed between the respective magnet-retaining apertures and the respective permanent magnets, a relatively high injection pressure is required. In this connection, in the rotor core formed by laminating magnetic sheets, when a portion of a material (hereinafter referred to as a marginal portion) defined between each magnet-retaining aperture and the outer circumferential surface of the rotor core becomes much thinner, each magnetic sheet may be deformed at the marginal portion so as to bulge outward due to a high injection pressure in the resinous material. It the rotor core is deformed in a manner as described above, during rotation of a rotor, a magnetic flux passing through the rotor core may be affected to degrade a rotational accuracy and/or the rotor core may come into contact with a stator core to be damaged and, thereby, it is concerned that an operational reliability of an electric motor is deteriorated